The present invention relates to an improved polyester multifilament yarn having a high modulus and low shrinkage which is useful for the reinforcement of rubber products such as tires.
Polyester yarns are widely utilized in various industrial rubber products such as tires, seat belts, conveyer velts, V-belts and hoses. In particular, treated cords manufactured by subjecting polyester yarns to latex- and heat-treatments have satisfactory dimensional stability and tenacity suitable for use as reinforcement tire cords.
U.S. Pat. No. 4,101,525 (Davis et al.) and U.S. Pat. No. 4,491,657 (Saito et al.) provide industrial polyester multifilament yarns having a high initial modulus and low shrinkage. However, such yarns provide reduced tenacity when they are converted to treated cords, as compared to conventional tire cords.
Accordingly, for the purpose of raising the tenacity of a polyester multifilament yarn, U.S. Pat. No. 4,690,866 (Kumakawa et al.) describes a method of making yarns using an ultra high viscosity polyester chip having an intrinsic viscosity (IV) of 1.2 or greater. The use of such a high IV polyester chip causes an increase in the spinning tension, which enhances both the orientation of the undrawn yarn and the formation of tie chains among crystals, thereby providing yarns of high tenacity, when converted to treated cords. However, such a high IV chip tends to exhibit a significant IV difference between its core and surface section, which results in excessive filament cuts during melt-spinning and drawing, yielding yarns having poor mechanical quality and appearance. In addition, as a high IV chip must be melt-spun at a high temperature, it tends to undergo partial decomposition and hydrolysis, giving yarns having an IV much lower than that of the original chip.
Accordingly, it is an object of the present invention to provide a polyester multifilament yarn having excellent properties which is capable of providing a treated cord having high tenacity and improved dimensional stability.
In accordance with one aspect of the present invention, there is provided a polyester multifilament yarn prepared by a method comprising the steps of: (A) melt-spinning a solid-phase polymerized polyester chip, composed of 90 mol % or more of ethylene terephthalate units as the recurring unit and having an intrinsic viscosity of 1.05 to 1.13, through a spinneret at a temperature ranging from 290 to 298xc2x0 C. to form a molten spun yarn,
(B) rapid-cooling and solidifying the spun yarn by passing it through a solidification zone,
(C) withdrawing the solidified yarn at such a rate that an undrawn yarn having a birefringence of 0.06 to 0.09 and a density of 1.360 to 1.375 is formed, and
(D) hot-drawing the yarn to a total draw ratio of 1.5 to 2.5; said yarn having (1) a unit filament thickness of 2.5 to 3.5 denier, (2) an IV of 0.94 to 1.00, (3) a DEG(diethylene glycol) content of 0.65 to 0.9 weight %, (4) a CEG(carboxyl end group) content of 23 eq./106 g or less, (5) a tenacity of 7.5 to 8.5 g/d, (6) an elongation of 13.0 to 16.0%, (7) a shrinkage of 4.0 to 7.0%, and (8) a silk factor (tenacity(g/d)xc3x97 elongation at break) of at least 27.